1. Field of the Invention
This invention relates generally to high-speed line printers, and more particularly to improved methods and apparatus for detecting paper jams in the forms stacker assembly of such printers.
2. Description of the Prior Art
In the impact printing field, a wide variety of printing techniques have been utilized in the past, including the present type of printer, which uses an endless steel character band having letters of the alphabet as well as numerals embossed thereon. The band is conventionally rotated between a drive pulley and an idler pulley. As the band is driven at high speeds adjacent a platen, a selected hammer contained within a bank or banks arranged parallel with respect to the band and platen is actuated to impact with a particularly desired character upon the band, thereby printing the character on forms passing through the print gate. Once printed, the forms are then fed into a stacker assembly which is typically located at the rear of the printer.
Conventional forms stacker assemblies fan-fold the forms such that they are stacked neatly within the stacker bin. It can be readily appreciated, however, that such fan-folding of the forms may lead to various jam conditions which require intervention on the part of the printer operator as well as considerable printer downtime. Controls are provided in typical forms stacker assemblies not only to indicate when the stacker bin is filled but also to hold the forms compacted in place. Furthermore, various sensors are provided which are capable of detecting when jams occur.
For example, in U.S. Pat. No. 4,054,283, issued Oct. 18, 1977 to Rayfield, a count jam circuit is provided. The stacker receives line strobe signals from the printer indicative of the rate at which paper is fed by the printer to the stacker. In such a manner, the line strobe signals control the rate of oscillation of the chute through which paper received by the stacker from the printer passes between two extreme positions. The distance between the two positions defines the oscillation stroke, which is determined by means of a manually-operable selector to be a function of the form length of the paper to be stacked. Paper is then stacked on a pedestal or elevator which is incrementally movable between top and bottom positions. A stack sensor is also provided which senses the top of the stack and causes the pedestal to move toward the bottom position so as to maintain the distance between the chute and the top of the stack relatively constant.
It has also been known in the past to vary the speed of the elevator in accordance both with the speed of the printing and with the thickness of the paper so that the elevator continuously moves at a predetermined speed. Other techniques involve the use of photocells for detecting the position of materials within the stacker bins. However, in U.S. Pat. No. 4,213,601, issued July 22, 1980 to Cattorini et al., and assigned to the assignee of the present invention, simple mechanical sensors are used to generate electrical pulses which operate the elevator in accordance with signals generated by the printer's microprocessor. Exemplary of other mechanical approaches are those printers which utilize a reed or lever to sense misfolded forms.
Another prior art optical system for jam detection is also utilized in the Impact 5000 Series printers manufactured by Documation Incorporated, now StorageTek Printer Corporation, a subsidiary of Storage Technology Corporation. In such printers, a photoemitter and sensor receiver are placed on opposite sides of the stack, and are offset such that a signal transmitted through the photoemitter is interrupted by every other fold in the forms. When a predetermined number of "top of forms" are observed by the printer without the optical sensor's changing state, a stacker jam is flagged. Similar optical sensors are utilized to directly monitor the paper stack in the Model 4248 printers manufactured by the IBM Corporation.
Each of the above described prior art jam detectors suffer from similar disadvantages. For example, the mechanical approaches can easily become entangled with the paper thereby resulting in either damaged forms or printer parts. The optical sensors' performance, on the other hand, has proven in the past to be marginal due to their relative position within the forms path. That is, typical prior art jam detectors having optical sensors position those sensors within the stacker bin. Such positioning of the sensors leads to the discovery of a problem long after jam begins. It would, therefore, be desirable to provide an improved jam detector which does not suffer from the above described disadvantages.